1. Field of the Invention
The present invention relates to a method of designing and forming a laminated chip type LC filter which may, for example, suitably be used as a noise filter for EMI and the like.
2. Description of the Prior Art
FIG. 1 is a table depicting equivalent circuit and insertion loss characteristics of various LC filters. As is evident from FIG. 1, capacitor and induction type filters have an insertion loss of approximately -20 dB/dec, the L type filter has an insertion loss of approximately -40 dB/dec, and the .pi. type filter has an insertion loss of approximately -60 dB/dec. The .pi. type filter, which has a relatively large number of elements, exhibits favorable characteristics as compared with the capacitor, induction and L type filters since the change in insertion loss of the .pi. type filter is very sharp with respect to frequency.
As an embodiment of the .pi. type LC filter, the structure illustrated in FIGS. 2, 3 and 4 is available, where FIG. 4 shows only a conductor line 4 and capacitor electrodes 5, 6, 7 & 8. (The applicant discloses this structure in U.S. Pat. No. 5,051,712.) This n type LC filter is made up of a laminated body having top and bottom dielectric layers 2, 3 formed on opposite sides of a magnetic material layer 1, and includes first and second external electrodes 10, 12 formed at opposite ends of the laminated body, and a third external electrode 12 formed at a center section of the laminated body. An inductor 13 is formed by at least one conductor line 4 located within the magnetic material layer 1 and having one end connected to the first external electrode 10 and another end to the second external electrode 11. A first capacitor 14 is formed by a pair of parallel capacitor electrodes 5, 6 located within the top dielectric layer 2. The capacitor electrode 5 and the capacitor electrode 6 are connected to the first external electrode 10 and the third external electrode 12, respectively. A second capacitor 15 is formed by a pair of parallel capacitor electrodes 7, 8 located within the bottom dielectric layer 3. The capacitor electrode 7 and the capacitor electrode 8 are connected to the second external electrode 11 and the third external electrode 12, respectively.
However, when the .pi. type LC filter as described above is, as shown in FIG. 5, mounted by soldering 24 on a printed circuit board 23 having surface electrodes 20, 21 and 22 formed thereon, the insertion loss characteristic can deteriorate in the high frequency range as shown in FIG. 6. This is due to the presence of lead inductance components L.sub.1 and L.sub.2 as shown in FIG. 7. The lead inductance component L.sub.1 is generated by the surface electrode 20, the first external electrode 10, the capacitor electrode 5, the capacitor electrode 6, the third external electrode 12 and the surface electrode 22, whereas the lead inductance component L.sub.2 is generated by the surface electrode 21, the second external electrode 11, the capacitor electrode 7, the capacitor electrode 8, the third external electrode 12 and the surface electrode 22. These lead inductance components L.sub.1 and L.sub.2 resonate with the capacitance C.sub.1 of the first capacitor 14 and the capacitance C.sub.2 of the second capacitor 15, respectively, thus causing the insertion loss deterioration shown in FIG. 6.